The present invention relates to the crystallinity of solid materials in general, and more particularly, to a method for reducing the crystallinity of nickel hydroxide powders precipitated from supersaturated aqueous solutions.
Inco Limited has developed an improved process for the direct production of nickel hydroxide by utilizing elemental nickel as the starting material. As opposed to conventional caustic precipitation methods, the elemental nickel process is environmentally friendly. See, for example, U.S. Pat. No 5,545,392 to Babjak et al.
The degree of crystallinity of certain solid materials produced by crystallization is critical. For example, the catalytic activity of some catalysts increase as their degree of crystallinity decreases. The same trend applies generally to the electrochemical activity of battery powders. Nickel hydroxide, used in power cells, is a typical example. It has been shown that the electrochemical activity of nickel hydroxide increases as its degree of crystallinity decreases. The degree of nickel hydroxide crystallinity is usually expressed in terms of xe2x80x9cFull Width Half Maximumxe2x80x9d (FWHM) of its x-ray defraction (xe2x80x9cXRDxe2x80x9d) [101] peak.
When the value of FWHM increases the degree of crystallinity decreases. For example, when the FWHM of nickel hydroxide is 0.1xc2x0, its crystallinity is very high and its electrochemical activity is low (below 50% of its theoretical value). When the nickel hydroxide""s FWHM is 0.9xc2x0 the degree of crystallinity is low and its electrochemical activity is high (close to its theoretical value of 289 mAh/g). Some publications give the crystallinity in terms of crystallite size (C.S.) which is estimated from the FWHM value. The crystallite size is an inverse function of FWHM (e.g. FWHM of 0.47xc2x0 corresponds to C.S. of about 25 nm, while FWHM of 0.95xc2x0 corresponds to C.S. of approximately 10 nm).
The methodology for modifying the crystallinity of powders precipitated/crystallized from aqueous solutions has not been clearly described in the literature. It appears that among various systems studied nickel hydroxide has received the most attention in terms of its synthesis and also its electrochemical testing. However, only a few literature sources discuss the effect of the conditions applied during the hydroxide synthesis on its crystallinity.
As alluded to above, most conventional commercial methods for synthesizing nickel hydroxide involve caustic precipitation from a nickel salt solution with a base in the presence of a complexing agent. Nickel sulfate, sodium hydroxide and ammonia are usually used as a nickel salt, a base and a complexing agent respectively. It has been shown that nickel hydroxide with a low degree of crystallinity can be obtained by precipitation from such system. For example, Japanese patent JP 06-340427 to Eiji et al. describes a process for precipitating nickel hydroxide, having FWHMxe2x89xa70.9xc2x0, from a nickel sulfate solution using a sodium hydroxide base in the presence of ammonia at 50xc2x0 C., pH 10.4 to 11.3, reactor residence time of 6.5-9 hours and impeller power input of 0.5-1.4 kW/m3. U.S. Pat. No. 5,702,844 to Bernard et al., demonstrates the precipitation of nickel hydroxide having a crystallite size generally below 10 nm and as low as 2.5 nm from a similar system at 36-50xc2x0 C.
It appears that the degree of supersaturation is high in these precipitation processes and presumably that may be an explanation why the product crystallinity is low.
In most processes the high degree of supersaturation cannot be achieved easily. In such situations controlling the degree of crystallinity becomes very difficult and very limited. The process described in U.S. Pat. No. 5,545,392 to Babjak et al. above may serve as an example. In this process nickel powder is directly converted into nickel hydroxide in an aqueous ammoniacal solution using oxygen as an oxidant. Nickel is dissolved and simultaneously precipitated as hydroxide. Since the two steps, i.e. the dissolution and the precipitation cannot be controlled independently the high degree of supersaturation cannot be achieved. As a consequence altering the degree of the product crystallinity is limited with such direct conversion processes.
There is provided a process for expeditiously altering the degree of crystallinity of nickel hydroxide where the concentration of the nickel hydroxide is precipitated/crystallized at a relatively low level of supersaturation. By generating and force feeding a large number of heteronuclei into the reaction system which by itself is incapable of generating the desired number of nuclei, the crystallinity of the resultant nickel hydroxide is dramatically reduced.